Rubber composition and pneumatic tire using said rubber composition

ABSTRACT

The present invention provides a rubber composition and a pneumatic tire using the rubber composition. The rubber composition which comprises: a rubber component comprising a styrene-butadiene copolymer rubber or a blend of the styrene-butadiene copolymer rubber and another conjugated diene base rubber with the styrene-butadiene copolymer rubber being 70% by weight or more of the entire blend, and the entire content of styrene is 30 to 40% by weight of the entire rubber component; silica in an amount of 10 to 60 parts by weight per 100 parts by weight of the rubber component; a specific silane coupling agent, i.e., a bis (alkoxysilylalkyl)polysulfide having a polysulfide chain in which the distribution of sulfur is specified, in an amount of 1 to 20% by weight of the amount of silica; and carbon black, the sum of the amount of silica and carbon black being 60 to 130 parts by weight per 100 parts by weight of the rubber component. The pneumatic tire has excellent wet skid resistance, excellent grip performance on a dry road surface, and excellent abrasion resistance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rubber composition and to a pneumatictire using the rubber composition, and more particularly, to a rubbercomposition and to a pneumatic tire using the rubber composition inwhich wet skid resistance and grip performance on a dry road surface areexcellent and abrasion resistance can be improved.

2. Description of the Related Art

Heretofore, silica has been used for the rubber composition of a tire soas to improve the performances thereof.

For example, Japanese Patent Application Laid-Open (hereinafter, “JP-A”)No. Sho-63-270751 and JP-A No. Sho-64-9248 disclose a rubber compositioncompounded with a predetermined amount of silica in order to obtain ahigh performance tire.

Further, JP-A No. Hei-3-252431, JP-A No. Hei-3-252433, and JP-A No.Hei-3-25431 disclose a pneumatic tire in which a rubber compositioncompounded with silica, silane coupling agent, and a specific polymer isused for a tread in order to improve wet skid resistance, rollingresistance, and abrasion resistance.

Several other tread rubber compositions compounded silica are proposedin order to improve the performances of a tire (e.g., JP-A No.Hei-4-224840, JP-A No. Hei-5-271477, JP-A No. Hei-5-51484, JP-A No.Hei-7-48476, and the like).

However, silica particles tend to cohere together due to hydrogenbonding of silanol groups which are functional groups on the surfaces ofthe silica particles. For improving the dispersion of silica particlesinto rubber, the mixing time must be increased. When dispersion ofsilica particles into rubber is insufficient, a problem arises in thatprocessability in processes such as extrusion and the like deterioratesdue to the increase in the Mooney viscosity.

Moreover, the surfaces of the silica particles are acidic. Therefore,there are problems in that basic substances used as vulcanizationaccelerators are absorbed such that vulcanization is not carried outsufficiently, and a sufficient modulus of elasticity is not obtained.

In order to solve these problems, various types of silane couplingagents have been developed. For example, use of a silane coupling agentas a reinforcing material is described in Japanese Patent ApplicationPublication (hereinafter, “JP-B”) No. Sho-50-29741. However, the use ofa silane coupling agent as a reinforcing material is still insufficientfor improving fracture properties, workability, and processability of arubber composition to high standards. Rubber compositions in which acombination of silica and silane coupling agent is used as a reinforcingmaterial are described in JP-B No. Sho-51-20208 and others. However,this method of using a combination of silica and silane coupling agentas a reinforcing material has a drawback in that flow of the uncuredcompounded rubber is markedly inferior and workability andprocessability deteriorate, although reinforcement of the compoundedrubber can be remarkably improved and fracture properties are improved.

The drawbacks of the conventional technologies in which silane couplingagents are used arise due to the following mechanism. When the mixingtemperature of rubber is low, the silanol group on the surface of thesilica does not react sufficiently with the silane coupling agent, andas a result, the sufficient reinforcing effect is not obtained.Moreover, some of the alcohol formed by the reaction of the silanolgroup on the surface of the silica and the silane coupling agent doesnot vaporize completely during mixing because of the low mixingtemperature, and the residual alcohol in the rubber vaporizes during anextrusion process so as to form blisters.

On the other hand, when mixing is conducted at high temperatures of 150°C. or more, the silanol group on the surface of the silica and thesilane coupling agent sufficiently react with each other, and as aresult, the reinforcing property is improved. Dispersion of the silicainto the rubber is also improved, a rubber composition having a goodabrasion resistance is obtained, and the formation of blisters in anextrusion process is suppressed. However, in this temperature range,gelation of the polymer caused by the silane coupling agent takes placesimultaneously, and the Mooney viscosity markedly increases. Thus,processing in later stages becomes impossible in actuality.

Therefore, when a silane coupling agent is used in combination withsilica, a multistep mixing must be conducted at a temperature lower than150° C., and marked decrease in productivity is inevitable. When themixing is conducted at a low temperature, dispersion of silica andcarbon black into the rubber is insufficient and abrasion resistancedeteriorates.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a rubbercomposition and a pneumatic tire using the rubber composition in whichgelation of a polymer due to a silane coupling agent during mixing athigh temperatures of 150° C. or more is suppressed without adeterioration in workability, in which wet skid resistance and gripperformance on a dry road surface are excellent, and in which abrasionresistance can be improved.

In order to achieve the above object, a rubber composition of thepresent invention which comprises: a rubber component comprising astyrene-butadiene copolymer rubber, or a blend of the styrene-butadienecopolymer rubber and another conjugated diene base rubber with thestyrene-butadiene copolymer rubber being 70% by weight or more of theentire blend, and the entire content of styrene is 30 to 40% by weightof the entire rubber component; silica in an amount of 10 to 60 parts byweight, preferably 20 to 50 parts by weight, per 100 parts by weight ofthe rubber component; in an amount of 1 to 20% by weight, preferably 3to 15% by weight, of the amount of silica, a silane coupling agentrepresented by following general formula (1):(C_(n)H_(2n+1)O)₃Si—(CH₂)_(m)—S_(y)—(CH₂)_(m)—Si(C_(n)H_(2n+1)O)₃   (1)(wherein n represents an integer of 1 to 3, m represents an integer of 1to 9, y represents a positive number of 1 or more which has adistribution), and in which the content of trisulfide silane is 20% ormore based on the entire polysulfide silane, and the content of highpolysulfide silane, in which y represents 5 or a number larger than 5,is 50% or less based on the entire polysulfide silane; and carbon black,the sum of the amount of silica and carbon black being 60 to 130 partsby weight.

Further, it is preferable that the content of trisulfide silane in thepolysulfide silane coupling agent molecule represented by the abovegeneral formula (1) is 30% or more based on the entire polysulfidesilane and that the content of high polysulfide silane, in which yrepresents 5 or a number larger than 5, is 40% or less based on theentire polysulfide silane.

Moreover, it is preferable that the amount of styrene-butadienecopolymer rubber obtained by emulsion polymerization is 40% by weight ormore of the percentage by weight of the styrene-butadiene copolymerrubber in the rubber component.

Furthermore, it is preferable that a nitrogen absorption specificsurface area (N₂SA) of the carbon black is 120 to 160 m²/g and a dibutylphthalate oil absorption (DBP) thereof is 120 to 150 ml/100 g.

Still further, the present invention relates to a pneumatic tire whichis manufactured by using the rubber composition for tread rubber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A rubber component of the rubber composition in the present inventioncomprises a styrene-butadiene copolymer rubber, or a blend of thestyrene-butadiene copolymer rubber and another conjugated diene baserubber with the styrene-butadiene copolymer rubber being 70% by weightor more of the entire blend, preferably a styrene-butadiene copolymerrubber obtained by emulsion polymerization in an amount of 40% by weightor more of the percentage by weight of the styrene-butadiene copolymerrubber, and the entire content of styrene is 30 to 40% by weight of theentire rubber component. When the percentage of styrene-butadienecopolymer rubber is less than 70% by weight of the entire rubbercomponent, wet skid resistance and grip performance on a dry roadsurface deteriorate. Further, when the entire content of styrene is lessthan 30% by weight of the entire rubber component, wet skid resistanceand grip performance o n a dry road surface deteriorate, and when theentire content of styrene exceeds 40% by weight thereof, abrasionresistance deteriorates. As the other conjugated diene base rubbers,polybutadiene, natural rubber, synthetic cis 1,4-polyisoprene rubber,butyl rubber, halogenated butyl rubber, and the like can be used.

The silica used in the present invention is a synthetic silicamanufactured by a precipitation process. Preferably, a synthetic silicathrough a precipitation process having a BET specific surface area of140 to 280 m²/g and a dibutyl phthalate oil absorption of 150 to 300ml/100 g is used. Specific examples of the silica include NIPSIL AQmanufactured by NIPPON SILICA INDUSTRIAL Co., Ltd.; ULTRASIL VN3 and BV3370GR manufactured by DEGUSSA AG., a German company; RP1165MP, Zeosil165GR, and Zeosil 175MP manufactured by RHÔNE-POULENC Co.; and Hisil233,Hisil210, Hisil255, Hisil2000 manufactured by PPG Co. (all trade names).However, the silica used in the present invention is not limited tothese examples. The amount of silica used is 10 to 60 parts by weight,preferably 20 to 50 parts by weight, per 100 parts by weight of theconjugated diene base rubber. When the amount of silica is less than 10parts by weight, wet skid resistance deteriorates. On the other hand,when the amount of silica exceeds 60 parts by weight, abrasionresistance deteriorates.

The silane coupling agent used in the present invention is a silanecoupling agent represented by following general formula (1):(C_(n)H_(2n+1)O)₃Si—(CH₂)_(m)—S_(y)—(CH₂)_(m)—Si(C_(n)H_(2n+1)O)₃   (1)(wherein n represents an integer of 1 to 3, m represents an integer of 1to 9, and y represents a positive number of 1 or more which has adistribution). It is necessary that the content of trisulfide silane is20% or more, preferably 30% or more, based on the entire polysulfidesilane and that the content of high polysulfide silane, in which yrepresents 5 or a number greater than 5, is 50% or less, preferably 40%or less. By using this silane coupling agent, the effect of suppressinggelation of a polymer during mixing at high temperatures of 150° C. ormore is obtained, and the reduction of productivity due to the increasein the Mooney viscosity can be prevented.

The amount of silane coupling agent used is 1 to 20% by weight,preferably 3 to 15% by weight, of the weight of silica. When the amountof silane coupling agent used is less than 1% by weight, a sufficientreinforcing effect is not obtained. On the other hand, when the amountof silane coupling agent exceeds 20% by weight of the weight of silica,the modulus of elasticity of a cured rubber increases too much.Therefore, such amounts are not preferable.

To effectively exhibit the characteristics of the rubber composition ofthe present invention, the mixing temperature is preferably 150° C. ormore and 180° C. or less. When the mixing temperature is less than 150°C., the silane coupling agent does not react sufficiently with thesilica, and blisters are formed during extrusion. On the other hand,when the temperature exceeds 180° C., gelation of the polymer takesplace such that the Mooney viscosity increases. Therefore, suchtemperatures are not preferable from the standpoint of processing.

The mechanism for preventing gelation of a polymer and improvingabrasion resistance at a mixing temperature of 150° C. or more isdescribed hereinafter on the basis of the results of studies andconsiderations of the results.

A silane coupling agent generally used in the tire industry (trade name:Si69, manufactured by DEGUSSA AG., a German company) was heated in anoven at 150° C. for 2 hours and cooled. Thereafter, the treated silanecoupling agent was analyzed by high performance liquid chromatography.It was confirmed from the results of the analysis that the componentshaving sulfur chains of —S₆— or longer in the molecule were decreased ascompared to the original material, and the free sulfur and componentshaving sulfur chains of —S₄— or shorter in the molecule were increasedas compared to the original material. In other words, it was thoughtthat the components having sulfur chains of —S₆— or longer in themolecule were decomposed by the heating at a high temperature. It can besurmised that gelation of a polymer takes place during mixing at a hightemperature because radicals are formed during the decomposition of thesilane coupling agent or because products formed by the decompositionwork as a source of sulfur. Therefore, it was believed that gelation ofa polymer is suppressed during mixing at temperatures of 150° C. or morewhen the silane coupling agent originally contains smaller amounts ofthe components having long sulfur chains in the molecule. As the resultof intensive studies in accordance with the above idea, it was foundthat, when the proportion of the components having short sulfur chainsin the molecule among the components having sulfur chains of variouslengths in the molecule was increased to a specific value or more,gelation of the polymer was actually suppressed. Moreover, dispersion ofsilica into rubber was improved because the reaction of the silanolgroup on the surface of the silica and the silane coupling agent tookplace sufficiently due to mixing at a high temperature, and abrasionresistance was improved.

Carbon black is used along with silica as a filler for the rubbercomposition of the present invention. The sum of the amount of carbonblack and silica is 60 to 130 parts by weight. When the sum of theamount of carbon black and silica is less than 60 parts by weight, thetensile strength at the time of cutting of a rubber is low and abrasionresistance deteriorates. On the other hand, when the sum of the amountof carbon black and silica exceeds 130 parts by weight, dispersion ofcarbon black and silica into rubber is insufficient and abrasionresistance deteriorates again.

The carbon black having a nitrogen absorption specific surface area(N₂SA) of 120 to 160 m²/g and a dibutyl phthalate oil absorption (DBP)of 120 to 150 ml/100 g is preferably used. DBP is determined inaccordance with ASTM D2414-93 and N₂SA is determined in accordance withASTM D4820.

Into the rubber composition of the present invention, compoundingingredients which are generally used such as antioxidants, zinc oxide,stearic acid, softeners, and the like can be used.

EXAMPLES

The present invention is described more specifically with reference tothe following Examples.

Various rubber compositions were prepared in accordance withformulations given in the following Tables 2 and 3. The silane couplingagents used in the formulations are expressed by the following formula:(C₂H₅O)₃Si(CH₂)₃—S_(y)—(CH₂)₃Si(C₂H₅O)₃,and —S_(y)— in this formula has the distribution shown in Table 1. Thedistributions of various sulfur chain components (—S_(y)—) shown inTable 1 were obtained by calculation from peak areas (%) obtained byhigh performance liquid chromatography (HPLC). The analysis by HPLC isdescribed in detail hereinafter.(Conditions of analysis by HPLC)

-   HPLC: manufactured by TOSOH CORPORATION, HLC-8020-   UV detector: manufactured by TOSOH CORPORATION, UV-8010 (254 nm)-   Recorder: manufactured by TOSOH CORPORATION, SUPER SYSTEM CONTROLLER    SC-8010-   Column: manufactured by TOSOH CORPORATION, TSK-gel ODS-80T_(M)CTR    (inner diameter: 4.6 mm, length: 10 cm) Temperature at the time of    measurement: 25° C.-   Concentration of sample: 6 mg/10 cc (6 mg per 10 cc of acetonitrile    solution-   Amount of sample injected: 20 μl-   Condition of elusion: flow rate of 1 cc/min

A sample was eluted for 2 minutes with a mixed solution of acetonitrileand water having a fixed composition of 1:1, and then with a mixedsolution having a varying composition with such a gradient that thesolution contained 100% of acetonitrile after 18 minutes.

TABLE 1 -S₅- or -S₂- -S₃- -S₄- -S₅- -S₆- -S₇- -S₈- -S₉- more sample 2.5315.85 23.77 24.27 18.33 10.24 3.83 1.18 57.85 A*¹⁾ sample 7.16 30.3329.38 18.29 8.24 3.28 0.96 2.36 33.13 B sample 17.64 44.14 23.40 8.491.92 1.06 3.37 0 14.83 C sample 8.1 59.0 18.7 14.2 0 0 0 0 14.2 D sample11.1 62.8 26.1 0 0 0 0 0 0 E sample 97.3 2.7 0 0 0 0 0 0 0 F

Samples A to F in Table 1 were obtained as follows.

Sample A

Si69, manufactured by DEGUSSA AG., a German company

Samples B and C

Samples B and C were synthesized in accordance with the method describedin JP-A No. Hei-7-228588 from anhydrous sodium sulfide and sulfur in thefollowing mol ratios:

sample B 1:2

sample C 1:1.5

Sample D

506 g (1 mol) of sample B which has a polysulfide distribution shown inTable 1 was weighed and charged into a 1 -liter flask. 78.7 g (0.3 mol)of triethyl phosphite was added dropwise into the flask through adropping funnel over 2 hours while stirring the solution within theflask. During this time, the temperature within the flask rose from 25°C. to 50° C. The stirring was conducted for another 3 hours and aportion of the solution was checked through gas chromatography. It wasfound that a peak assigned to triethyl phosphite was diminished and thatthe reaction took place. Table 1 shows the results of measurement ofpolysulfide distributions in the obtained composition through liquidchromatography. It shows that high polysulfide portions selectivelyreacted with the phosphite.

Sample E

538 g (1 mol) of silane (sample A: Si69 manufactured by DEGUSSA AG., aGerman company), which has the polysulfide distribution shown in Table 1and which has an average of four sulfur atoms per polysulfide chain, wasweighed and charged into a 1-liter flask. 166.2 g (1 mol) of triethylphosphite was added dropwise into the flask through a dropping funnelover 2 hours while stirring the solution within the flask. During thistime, the flask was cooled by water in order to maintain the temperaturetherewithin at 50° C. or lower. Next, the flask was heated and stirredfor 3 hours at 40 to 50° C. Thereafter, sample E was obtained in thesame way as sample D.

Sample F

Sample F was synthesized in accordance with the method described in JP-ANo. Hei-8-259739.

Various types of rubber compositions in Examples and ComparativeExamples were prepared by using the obtained samples. Then, 225/50R16size tires having treads formed by respective rubber compositions inExamples and Comparative Examples were manufactured and tested by usingan 8J-16 rim. The obtained rubber compositions were evaluated withrespect to Mooney viscosity in accordance with the following method.Further, the manufactured tires were evaluated with respect to abrasionresistance, grip performance on a dry road surface, and wet skidresistance in accordance with the following methods.

(1) Mooney Viscosity

Mooney viscosity was measured in accordance with the method of JapaneseIndustrial Standard K6300 for a time of 4 minutes at a temperature of130° C. after preheating for 1 minute. The obtained result is expressedas an index with Comparative Example 1 being 100. The smaller the index,the lower the Mooney viscosity and the better the processability.

(2) Abrasion Resistance

Four test tires were placed on a 2000 cc passenger vehicle. After thevehicle was run about 30,000 km, the depth of a groove remained at thetire was measured. The abrasion resistance was obtained in accordancewith the following formula: {(running distance (km) of test tire)/(depth(mm) of initial groove−depth (mm) of groove remained at the tire afterthe test)/{(running distance (km) of tire of Comparative Example1)/(depth (mm) of initial groove−depth (mm) of groove remained at tireof Comparative Example 1 after the test)}. The larger the numericalvalue of the index, the better the abrasion resistance.

(3) Grip Performance on Dry Road Surface

A vehicle on which test tires were placed was run on a circuit and theaverage lap time of 8 to 10 laps was measured. The obtained result isexpressed as an index with the reciprocal of the lap time of ComparativeExample 1 being 100. The larger the numerical value of the index, thebetter the grip performance.

(4) Wet Skid Resistance

On a wet concrete road surface on which there is water of 3 mm depth,rapid braking is applied at a speed of 80 km/hour. The distance betweenthe locking of wheels and the stopping thereof was measured, and the wetskid resistance of the test tire was evaluated in accordance with thefollowing formula:{(stopping distance of tire of Comparative Example 1)/(stopping distanceof test tire)}×100.The larger the numerical value of the index, the better the wet skidresistance.

The obtained results are illustrated in the following Tables 2 and 3.

TABLE 2 Example 1 2 3 4 5 6 7 Comparative Example 1 2 3 Formulation(parts by weight) *1712*¹ 27.5 27.5 27.5 27.5 27.5 27.5 27.5 20.6 — 27.50120*² 110.0 110.0 110.0 110.0 110.0 110.0 110.0 — — 110.0 0202*³ — — —— — — — 30 75 — TAFDEN 2530*⁴ — — — — — — — 75.6 — — BR31*⁵ — — — — — —— 34.38 carbon type N134 N134 N134 N134 N134 N134 N134 N134 N234 N134black*⁶ amount 70 70 70 70 70 70 70 70 100 70 silica*⁷ 30 30 30 30 30 3030 30 10 40 silane type A A B C D E F C C C coupling amount 3.0 3.0 3.03.0 3.0 3.0 3.0 3.0 1.0 4.0 agent aromatic oil 12.5 12.5 12.5 12.5 12.512.5 12.5 23.8 40.6 12.5 stearic acid 1 1 1 1 1 1 1 1 1 1 antioxidant*⁸1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 zinc oxide 3.0 3.0 3.0 3.0 3.03.0 3.0 3.0 3.0 3.0 vulcanization 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.60.6 accelerator DM*⁹ vulcanization accelerator 0.3 0.3 0.3 0.3 0.3 0.30.3 0.3 0.3 0.3 DPG*¹⁰ vulcanization accelerator 2.3 2.3 2.3 2.3 2.3 2.32.3 2.3 2.3 23 NS*¹¹ sulfur 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9master batch 142 164 166 168 165 166 164 166 164 167 temperature (° C.)Results of evaluation entire content of styrene 32.7 32.7 32.7 32.7 32.732.7 32.7 31.1 34.5 32.7 Mooney viscosity (index) 100 168 98 91 90 92 8487 97 97 abrasion resistance 100 — 108 104 106 105 87 106 118 107(imdex) dry grip (index) 100 — 103 103 104 103 93 102 106 106 wet grip(index) 100 — 103 104 104 104 99 102 102 109 mastermatch temperature: ameasured temperature of masterbatch immediately after mixing

In Table 2, a tire of Comparative Example 2 cannot be manufactured dueto the increase in Mooney viscosity.

TABLE 3 Example 8 9 10 Comparative Example 4 5 6 7 8 9 Formulation(parts by weight) 1712*¹ 27.5 — 82.5 — 27.5 27.5 — — — 0120*² 110.0 40.555 — 110.0 110.0 — — — 0202*³ — 70 — 75 — — 75 75 68 TAFDEN 2530*⁴ — — —— — — — — — BR31*⁵ — — — 34.38 — — 34.38 34.38 44 carbon type N134 N134N134 N234 N134 N134 N134 N134 N134 black*⁶ amount 70 70 70 100 65 20 3550 35 silica*⁷ 60 30 30 0 70 30 25 60 25 silane type C C C — C C C C Ccoupling amount 12.0 3.0 3.0 — 7.0 7.5 3.0 9.0 3.0 agent aromatic oil12.5 39.5 12.5 40.6 12.5 12.5 40.6 40.6 38 stearic acid 1 1 1 1 1 1 1 11 antioxidant*⁸ 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 zinc oxide 3.0 3.03.0 3.0 3.0 3.0 3.0 3.0 3.0 vulcanization 0.6 0.6 0.6 0.6 0.6 0.6 0.60.6 0.6 accelerator DM*⁹ vulcanization accelerator 0.3 0.3 0.3 0.3 0.30.3 0.3 0.3 0.3 DPG*¹⁰ vulcanization accelerator 2.3 2.3 2.3 2.3 2.3 2.32.3 2.3 2.3 NS*¹¹ sulfur 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 masterbatch 164 166 163 164 165 165 163 166 162 temperature (° C.) Results ofevaluation entire content of styrene 32.7 41.2 28.1 34.5 32.7 32.7 34.534.5 31.3 Mooney viscosity (index) 99 93 94 94 102 72 84 96 89 abrasionresistance 104 94 108 107 96 82 102 112 107 (index) dry grip (index) 104106 96 104 103 96 101 104 98 wet grip (index) 110 105 94 98 112 92 101113 96 *¹bonded styrene, 23.5% by weight, 37.5 parts oil-extended;emulsion polymerization styrene-butadiene copolymer rubber (SBR)(manufactured by JAPAN SYNTHETIC RUBBER Co., Ltd.) *²bonded styrene,35.0% by weight, 37.5 parts oil-extended, emulsion polymerizationstyrene-butadiene copolymer rubber (SBR) (manufactured by JAPANSYNTHETIC RUBBER Co., Ltd.) *³bonded styrene, 46.0% by weight, notoil-extended, emulsion polymerization styrene-butadiene copolymer rubber(SBR) (manufactured by JAPAN SYNTHETIC RUBBER Co., Ltd.) *⁴bondedstyrene, 25.0% by weight, 37.5 parts oil-extended, emulsionpolymerization styrene-butadiene copolymer rubber (SBR) (manufactured byASAHI CHEMICAL INDUSTRY Co., Ltd.) *⁵Ni catalyst (37.5 partsoil-extended) high cis-butadiene rubber (manufactured by JAPAN SYNTHETICRUBBER Co., Ltd.) *⁶N134(N₂SA: 142 m²/g, DBP: 127 ml/100 g), N234(N₂SA:126 m²/g, DBP: 125 ml/100 g) (manufactured by ASAHI CARBON Co., Ltd.)*⁷NIPSIL AQ (manufactured by NIPPON SILICA INDUSTRIAL Co., Ltd.)*⁸N-(1,3-dimethyl-butyl)-N′-phenyl-p-phenylenediamine*⁹dibenzothiazylsulfide *¹⁰diphenylguanidine*¹¹N-tert-butyl-2-benzothiazolylsulphenamide mastermatch temperature: ameasured temperature of masterbatch immediately after mixing

Because the rubber composition of the present invention uses a silanecoupling agent having a specific distribution of polysulfide, gelationof a polymer due to the silane coupling agent is suppressed duringmixing at high temperatures of 150° C. or more without a decrease inworkability. The rubber composition is widely used for various types ofpneumatic tires having excellent wet skid resistance, grip performanceon a dry road surface, and abrasion resistance.

1. A rubber composition which comprises: a rubber component comprising astyrene-butadiene copolymer rubber, or a blend of the styrene-butadienecopolymer rubber and another conjugated diene based rubber with thestyrene-butadiene copolymer rubber being 70% by weight or more of theentire blend, and the entire content of styrene is 30 to 40% by weightof the entire rubber component; silica in an amount of 10 to 60 parts byweight per 100 parts by weight of the rubber component; in an amount of1 to 20% by weight of the amount of silica, a silane coupling agentrepresented by following general formula (I):(C_(n)H_(2n+1)O)₃Si—(CH₂)_(m)—S_(y)—(CH₂)_(m)—Si(OC_(n)H_(2n+1))₃   (1) wherein n represents an integer of 1 to 3, m represents an integer of 1to 9, y represents a positive number of 1 or more and has adistribution, and in which the content of trisulfide silane component,where y is 3, is 20% or more based on the entire amount of the silanecoupling agent, and the content of high polysulfide silane components,where y is 5 or a number larger than 5, is 50% or less based on theentire amount of silane coupling agent; and carbon black, the sum of theamount of silica and carbon black being 60 to 130 parts by weight per100 parts by weight of the rubber component.
 2. A rubber compositionaccording to claim 1, wherein the amount of silica is 20 to 50 parts byweight per 100 parts by weight of the rubber component.
 3. A rubbercomposition according to claim 1, wherein the amount of silane couplingagent is 3 to 15% by weight of the amount of silica.
 4. A rubbercomposition according to claim 1, wherein the content of the trisulfidesilane component, where y is 3, in the sulfide silane coupling agentmolecule represented by general formula (1) is 30% or more based on theentire amount of the silane coupling agent, and the content of the highpolysulfide silane components, where y represents 5 or a number largerthan 5, is 40% or less based on the entire amount of the silane couplingagent.
 5. A rubber composition according to claim 1, wherein the amountof styrene-butadiene copolymer rubber obtained by emulsionpolymerization is 40% by weight or more of the percentage by weight ofthe styrene-butadiene copolymer rubber in the rubber component.
 6. Arubber composition according to claim 1, wherein a nitrogen absorptionspecific surface area (N₂SA) of the carbon black is 120 to 160 m²/g anda dibutyl phthalate oil absorption (DBP) of the carbon black is 120 to150 ml/100 g.
 7. A pneumatic tire which is manufactured by using therubber composition described in claim 1 for tread rubber.
 8. A pneumatictire which is manufactured by using the rubber composition described inclaim 4 for tread rubber.
 9. A pneumatic tire which is manufactured byusing the rubber composition described in claim 5 for tread rubber. 10.A pneumatic tire which is manufactured by using the rubber compositiondescribed in claim 6 for tread rubber.
 11. A rubber compositionaccording to claim 1, wherein the content of the trisulfide silanecomponent, where y is 3, in the silane coupling agent moleculerepresented by general formula (1) is 25% or more based on the entireamount of the silane coupling agent.
 12. A rubber composition whichcomprises: a rubber component comprising a styrene-butadiene copolymerrubber, or a blend of the styrene-butadiene copolymer rubber and anotherconjugated diene based rubber with the styrene-butadiene copolymerrubber being 70% by weight or more of the entire blend, and the entirecontent of styrene is 30 to 40% by weight of the entire rubbercomponent; silica in an amount of 10 to 60 parts by weight per 100 partsby weight of the rubber component; mixing with the rubber component andthe silica, a silane coupling agent represented by following generalformula (1) in an amount of 1 to 20% by weight of the amount of silica:(C_(n)H_(2n+1)O)₃Si—(CH₂)_(m)—S_(y)—(CH₂)_(m)—Si(C_(n)H_(2n+1)O)₃   (1)wherein n represents an integer of 1 to 3, m represents an integer of 1to 9, and in which y ranges from 2 to 9 such that the silane couplingagent represents a mixture of polysulfide silanes, and in which thecontent of disulfide silane component, where y is 2, is 8.1% or morebased on the entire amount of the silane coupling agent, the content oftrisulfide silane component, where y is 3, is 44.14% or more based onthe entire amount of the silane coupling agent, and the content of highpolysulfide silane components, where y is 5 or a number greater than 5,is 50% or less based on the entire amount of the silane coupling agent;and carbon black, the sum of the amount of silica and carbon black being60 to 130 parts by weight per 100 parts by weight of the rubbercomponent.
 13. A rubber composition according to claim 12, wherein theamount of silica is 20 to 50 parts by weight per 100 parts by weight ofthe rubber component.
 14. A rubber composition according to claim 12,wherein the amount of silane coupling agent is 3 to 15% by weight of theamount of silica.
 15. A rubber composition according to claim 12,wherein the content of the high polysulfide silane components, where yrepresents 5 or a number larger than 5, is 40% or less based on theentire amount of the silane coupling agent.
 16. A rubber compositionaccording to claim 12, wherein the amount of styrene-butadiene copolymerrubber obtained by emulsion polymerization is 40% by weight or more ofthe percentage by weight of the styrene-butadiene copolymer rubber inthe rubber component.
 17. A rubber composition according to claim 12,wherein a nitrogen absorption specific surface area (N₂SA) of the carbonblack is 120 to 160 m²/g and a dibutyl phthalate oil absorption (DBP) ofthe carbon black is 120 to 150 ml/100 g.
 18. A pneumatic tire which ismanufactured by using the rubber composition described in claim 12 fortread rubber.
 19. A pneumatic tire which is manufactured by using therubber composition described in claim 15 for tread rubber.
 20. Apneumatic tire which is manufactured by using the rubber compositiondescribed in claim 16 for tread rubber.
 21. A pneumatic tire which ismanufactured by using the rubber composition described in claim 17 fortread rubber.